1. Field of the Invention
This invention relates to unitary, self-generating reference gas sensors, useful to detect SO.sub.2, CO.sub.2 and NO.sub.2 gases.
2. Description of the Prior Art
The requirements for monitoring and controlling stack gas pollutants have resulted in the development of solid electrolyte gas sensors having electrolyte compostions uniquely responsive to gases such as SO.sub.2, CO.sub.2 and NO.sub.2. These sensors are electrochemical concentration cells which sense the equilibrium of a gas species of interest and generate an EMF signal corresponding to the difference in partial pressure of the gas species across the solid electrolyte sensor. Typically, the solid state sensor includes an ion conductive solid electrolyte with electrodes disposed on its opposite surfaces. The stack gas, or monitored gas stream, contacts a sensing electrode, while the opposite electrode serves as a reference electrode which is contacted with a reference gas stream. Conventional solid electrolyte compositions require operating temperatures of between 200.degree. C. and 900.degree. C. to exhibit the desired ion conductivity to generate a suitable EMF signal.
In the past, a major problem with these devices was isolation of the monitored gas from the reference gas, to prevent unpredictable drift in the measurement signal. Hirayama et al., in U.S. Pat. No. 4,377,460, solved this sealing problem by using a closed end, gas impermeable, mullite (3Al.sub.2 O.sub.3. 2SiO.sub.2) tube, which acts as an alkali ion conductive membrane at high temperatures. The mullite tube, like most ceramics, incorporates some alkali oxide impurities, such as K.sub.2 O, making it a K.sup.+ ionic conductor at high temperatures. This tube was used to separate the two gas streams and provide two identical alkali ion conductive half cells secured to opposite sides of the mullite.
The two, alkali ion conductive solid electrolyte disc used in each half cell of the Hirayama et al. design, to monitor SO.sub.2, CO.sub.2 or NO.sub.2, were made of K.sub.2 SO.sub.4, Na.sub.2 CO.sub.3, or NaNO.sub.3 respectively. A platinum electrode was attached to one side of each half cell electrode. In the case of a SO.sub.2 +O.sub.2 reference gas stream, this provided the cell assembly: EQU SO.sub.2 +O.sub.2 Reference Gas, Pt.vertline.K.sub.2 SO.sub.4 .vertline.Mullite.vertline.K.sub.2 SO.sub.4 .vertline.Pt,SO.sub.2 +O.sub.2 Flue Gas
Lin et al., in U.S. Pat. No. 4,427,525, taught a somewhat similar system using calcia stabilized zirconia as the solid electrolyte. These sensor designs, however, are complicated to make and operate. Also, this use of a SO.sub.2 +O.sub.2 reference gas stream is inconvenient and expensive, since a constant supply of certified tank gas is required.
Several instances of simplified, unitary gas sensors have been disclosed in the art. Isenberg, in U.S. Pat. No. 3,915,830, relating to O.sub.2 sensors, taught hermetically encapsulating a metal/metal oxide reference medium, such as nickel/nickel oxide, exhibiting a stable oxygen activity, within a small, stabilized zirconia solid electrolyte disc. A metal electrode is attached to the outside of the solid electrolyte and is in electronic communication with the encapsulated reference medium. Sealing other reference media, such as oxygen gas or air within the solid electrolyte is also mentioned. Inoue et al., in U.S. Pat. No. 4,399,017, taught encapsulation of an electrode within a microporous, stabilized zirconia solid electrolyte. A second electrode is attached to the outside of the solid electrolyte, and the whole covered with porous ceramic. Upon application of a D.C. current, migration of oxygen ions, and diffusion of oxygen gas through the microporous solid electrolyte, can establish a reference partial pressure of oxygen at the interface between the microporous solid electrolyte and the encapsulated electrode, to enable measurement of oxygen gas content in flue gas.
Pebler, in U.S. Pat. No. 4,394,240, taught triangular, combination electrochemical cells, which form an internal cavity which contains a common internal gas forming reference. In the triangular configuration, two sides are made of stabilized zirconia, oxygen ion conductive solid electrolyte, and the third side can be made of K.sub.2 SO.sub.4 when SO.sub.3 or SO.sub.2 gases are to be measured. Reference electrodes are disposed on the inside electrolyte walls of the triangular configuration and sensing electrode are disposed on the outside electrolyte walls. The measuring concept utilizes heating a central, enclosed, MgSO.sub.4, MnSO.sub.4 or Ag.sub.2 SO.sub.4 reference material, which provides SO.sub.3 on decomposition. This reference material must be kept sealed from K.sub.2 SO.sub.4 electrolyte, because of the possible reaction of these two components at high temperatures.
None of these designs provide a simple, inexpensive construction that would be effective to measure SO.sub.2, CO.sub.2 or NO.sub.2 content of flue gases. It is an object of this invention to provide such a construction.